At around 2:00 p.m., sodium hydroxide began leaking from a neutralisation tower at a water treatment plant. This tower serves as the chlorine treatment system in the event of a leak in the storage or filling facilities. A total of 7 t of chlorine is stored at the plant to treat the drinking water supplied to the Greater Belfort urban region and the town of Montbéliard.

The sodium hydroxide solution (between 500 l and 1 m³) spread through the tower’s air extraction system and then discharged into the chlorine storage and distribution room. It spilt onto the road and along the highway via a gutter, disrupting traffic. Firefighters laid down absorbents and rinsed the road to dilute the sodium hydroxide residue. A second leak, occurring at the same time as the first one, sprang on the sodium hydroxide storage tank’s manhole and flowed into a secondary containment (flow rate: 1–3 l per minute).

The inspection authorities for classified facilities arrived the following day and saw that the road was stained with traces of crystallised sodium hydroxide. The operator excavated 6 m³ of contaminated soil and sent it to another water treatment plant which was not authorised to receive such materials but, according to the operator, posed less risk. The soil was stored in a tarpaulin-covered skip pending its treatment. The operator characterised the contaminated soil to determine the appropriate treatment method. The excavated land was backfilled with topsoil.

The neutralisation tower was returned to service five days later. The production of drinking water was not halted.

According to the operator, the leak occurred during a weekly test of the neutralisation tower. Sodium hydroxide had crystallised, forming a clog in the circuit that recirculates sodium hydroxide to the storage tank. The series of tests then caused the level of sodium hydroxide in the tower to increase until it reached the air extraction circuit.

Following this accident, the operator changed the dip tube on the sodium hydroxide recirculation system to prevent clogs from crystallisation. He repaired the storage tank’s manhole and checked the air heater system. He proposed improvements to, amongst other things:

• monitor the level of sodium hydroxide in the tank;
• prevent crystallisation;
• quickly return the neutralisation tower to service following an incident;
• contain accidental spills should they occur.

An exercise scenario is scheduled for the following year to assess how well the operator manages and responds to accidents.